Image processing often requires that two or more images from the same source or from different sources be “registered,” i.e., aligned, so that they occupy the same image space. That is, image registration comprises the process of identifying a mapping, or correspondence, between points (e.g., pixels or voxels) in a first image and points (e.g., pixels or voxels) in a second image. Once such a mapping has been accomplished, the images can be said to occupy the same image space. There are many techniques known in the art for registering images, including techniques for one, two, and three dimensional images.
Once aligned to the same image space, the aligned images can be useful in many applications. One such possible application is in medical imaging. For example, an image produced by positron emission tomography imaging (“PET”), and an image produced by computerized axial tomography (“CAT” or “CT”) can be registered, i.e., aligned, to accurately depict an area of the body. This technique may be applied to images from film, three dimensional gels, electronic portal imaging devices (EPID), digital radiography (DR) devices, computed radiography (CR) devices, and many other image sources. Additionally, a single alignment may be applied to multiple target images.
Another application of image registration is for quality assurance measurements. For example, the practice of radiation oncology often requires image treatment plans to be compared to acquired quality assurance images to determine whether the treatment plans are being executed accurately. A dose distribution treatment is planned and represented in an image (a “plan image” or “reference image”). An actual dose distribution associated with the planned distribution is then executed and captured in a second image (the “measured image” or “target image”). Next, the plan image is registered (i.e., aligned) with the measured image using conventional image registration techniques. Once the plan image and the measured image are registered in the same image space, known techniques can be used to measure the goodness of fit between the two aligned images. Goodness-of-fit measurements are useful for indicating differences between the planned dose distribution and the measured dose distribution.
From goodness of fit measurements, a level of accuracy of the actually delivered dose distribution can be determined relative to its associated planned dose distribution. Examples of techniques for comparing registered images to quantitatively evaluate the accuracy of planned dose distributions are provided in D. A. Low, W. B. Harms, S. Mutic, and J. A. Purdy, “A technique for the quantitative evaluation of dose distributions,” Med. Phys. 25, 656–661 (May 1998) (hereinafter “Low et al.”), fully incorporated by reference herein, and Nathan L. Childress and Isaac I. Rosen, “The Design and Testing of Novel Clinical Parameters for Dose Comparison, Int. J. Radiation Oncology Biology Physics, Vol. 56, No. 5, pp. 1464–1479 (2003), also fully incorporated by reference herein. As described by Low et al., dose differences and distance-to-agreement measurements are obtained from registered dose distribution images and used to calculate numerical quantifications of the goodness of fit between the measured and planned dose distributions represented in the registered images. Distance to Agreement (DTA) is the distance between a reference point (e.g., a pixel) in the measured image and the nearest point in the planned image that exhibits the same dosage value to a specified precision. Dose difference is the difference in dosage values (often represented by pixel intensities) between points in the plan image and the measured image.
As discussed by Low et al., the dose difference and DTA between different points located in a common image space are capable of graphical representation. FIGS. 1A and 1B illustrate geometric representations of the dose difference and DTA for a particular reference point 12 of the measured image and a particular target point 14 of the plan image located in a common image space.
FIG. 1A illustrates use of each of the dose difference and DTA tests to determine whether images are satisfactorily aligned. Reference point 12 is located at the origin of a graph 10 representing the image space. The x and y axes 16 and 18 of graph 10 represent the spatial location of target point 14. A third, or δ, axis 20 of graph 10 represents the dose difference 22 between a measured dose distribution represented at point 12 and a planned dose distribution represented at point 24.
A comparison of the location of reference point 12 and target point 14 on graph 10 can be made to determine whether the DTA 26 of points 12 and 14 exceeds a predetermined DTA criterion 30. DTA criterion 30 is represented by a circle 32, where the radius of circle 32 is equal to DTA criterion 30. If target point 14 lies within the circle 32, then DTA 26 meets DTA criterion 30. Similarly, if a line can be drawn representing dose difference 22 whose length is less than dose difference criterion 34, then target point 14 passes the dose distribution test.
Dose difference 22 and DTA 26 can be used together to evaluate the planned dose distribution in relation to the measured dose distribution. FIG. 1B illustrates a composite acceptance criterion 40 in the form of an ellipsoid that simultaneously considers the dose-difference criterion 34 and the DTA criterion 30 to determine whether the goodness of fit between the measured and planned images is at an acceptable level of accuracy. If any portion of the planned dose distribution 24 intersects the ellipsoid, the planned dose distribution 24 is determined to pass the composite acceptance criterion 40, i.e., planned dose distribution 24 has an acceptable level of accuracy in relation to the measured dose distribution.
Equations 1–7 below provide the basis for composite acceptance criterion 40. In Equations 1–7, rm denotes the position of reference point 12; rc denotes the position of target point 14; Dm denotes the dose distribution at reference point 12; Dc denotes the dose distribution at target point 14; Δdm denotes DTA criterion 30; and ΔDm denotes dose-difference criterion 34.
Equations 1–3 define composite acceptance criterion 40. Equation 1 defines the surface of the composite acceptance criterion 40 shown in FIG. 1B. Further, as will be understood by those skilled in the art, Equations 2 and 3 make clear that the ellipse defined by Equation 1 depends on DTA and dose difference calculations respectively.
      Equation    ⁢                  ⁢    1    ⁢          :                          ⁢          1      =                                                                  r                2                            ⁡                              (                                                      r                    m                                    ,                                      r                    c                                                  )                                                    Δ              ⁢                                                          ⁢                              d                M                2                                              +                                                    δ                2                            ⁡                              (                                                      r                    m                                    ,                                      r                    c                                                  )                                                    Δ              ⁢                                                          ⁢                              D                M                2                                                              Equation    ⁢                  ⁢    2    ⁢          :                          ⁢                  r        ⁡                  (                                    r              m                        ,                          r              c                                )                    =                                            r            c                    -                      r            m                                            Equation    ⁢                  ⁢    3    ⁢          :                          ⁢                  δ        ⁡                  (                                    r              m                        ,                          r              c                                )                    =                                    D            c                    ⁡                      (                          r              c                        )                          -                              D            m                    ⁡                      (                          r              m                        )                              
Composite acceptance criterion 40 can be used to calculate numerical quantifications of the goodness of fit between planned and calculated dose distributions. More specifically, a gamma index (γ) is calculated at each point in the plane defined by circle 32 for the reference point 12 using Equations 4–7.
      Equation    ⁢                  ⁢    4    ⁢          :                          ⁢                  γ        ⁡                  (                      r            m                    )                    =              min        ⁢                  {                      Γ            ⁡                          (                                                r                  m                                ,                                  r                  c                                            )                                }                ⁢                  ∀                      {                          r              c                        }                                    Equation    ⁢                  ⁢    5    ⁢          :                          ⁢                  Γ        ⁡                  (                                    r              m                        ,                          r              c                                )                    =                                                                  r                2                            ⁡                              (                                                      r                    m                                    ,                                      r                    c                                                  )                                                    Δ              ⁢                                                          ⁢                              d                M                2                                              +                                                    δ                2                            ⁡                              (                                                      r                    m                                    ,                                      r                    c                                                  )                                                    Δ              ⁢                                                          ⁢                              D                M                2                                                              Equation    ⁢                  ⁢    6    ⁢          :                          ⁢                  r        ⁡                  (                                    r              m                        ,                          r              c                                )                    =                                            r            c                    -                      r            m                                            Equation    ⁢                  ⁢    7    ⁢          :                          ⁢                  δ        ⁡                  (                                    r              m                        ,                          r              c                                )                    =                                    D            c                    ⁡                      (                          r              c                        )                          -                              D            m                    ⁡                      (                          r              m                        )                              
Accordingly, a planned dose distribution is acceptable when the gamma index (γ) is less than or equal to one (γ(rm)≦1) and unacceptable when the gamma index (γ) calculated in Equations 4 and 5 is greater than one (γ(rm)>1). Those skilled in the art will understand that, using the above equations, planned (i.e., calculated) dose distributions can be evaluated by comparing the goodness of fit between planned dose distributions and measured dose distributions, as represented in plan and measured images that have been registered, i.e., aligned.
In order for evaluative comparison techniques, including the techniques discussed by Low et al., to return reliable and helpful evaluation data, measured and calculated dose distribution images should be aligned as precisely as possible. Without accurate image registrations, errors will be introduced into image comparisons. Such errors are particularly undesirable in the field of radiation oncology, which depends on an accurate comparison of a plan image with a measured images to ensure that patients will receive the proper radiation doses during a course of radiation therapy.
However, image registration is especially problematic in the field of radiation oncology due to the common use of mega-voltage beams for radiation therapy. Images produced from mega-voltage beams tend to have poor resolutions, which render conventional image registration techniques ill-suited for achieving precise alignment of the images for several reasons. For example, cross-correlation alignment techniques do not reliably minimize overall image differences because differences in high-gradient areas may be magnified by small alignment errors and result in large translational shifts to compensate. Other existing techniques rely on the identification of structures or landmark points to use for alignment. However, these techniques are also ill-suited to align mega-voltage images because the poor resolution of the images makes selection of optimum matching points problematic, whether the selection is done manually or automatically. In sum, existing image alignment techniques return suboptimal alignments of poor resolution images because boundaries and landmarks are not well defined in the poorly focused images. Without precise image registration, mega-voltage radiation treatments cannot be reliably evaluated. Thus, it would be desirable to be able to accurately and precisely optimize image alignments, including alignments of mega-voltage images having poor resolutions.